A power-assisted rack and pinion steering unit has a number of internal oil seals. If oil leakage is present, one or more of the seals may require replacement. It is much less expensive to remove and replace the seals than to replace the entire power steering unit. However, it is usually difficult to determine which oil seals have an oil leakage problem. External oil leakage in some oil seals can be found by visual inspection; but oil leakage in most of the oil seals, such as the internal high pressure rack oil seal, cannot be determined by visual inspection.
One auto manufacturer recommends testing the rack and pinion steering unit for oil leakage while leaving the rack and pinion unit in place in the vehicle. However, in this instance it is difficult to reach the oil seals. If the power steering unit is removed from the vehicle, there is no method presently available for effectively testing all of the oil seals in the unit for leakage. The oil seals cannot be effectively tested absent the steering resistance of the wheels and the weight of the vehicle acting against turning of the power steering unit.
In a power-assisted rack and pinion steering unit, the rack is mounted in a long tubular portion of the housing. A power cylinder within the housing has a piston connected to the rack, and a rotary control valve in a pinion portion of the housing controls the flow of power steering fluid to the piston. When the steering wheel is turned, the resistance of the wheels and the weight of the vehicle cause a torsion bar in the steering unit to deflect. This deflection changes the position of the valve spool and sleeve ports, directing fluid under pressure through oil gallies to the appropriate end of the power cylinder. The difference in pressure forces on the piston which helps in moving the rack to assist the turning effort. The oil in the opposite end of the power cylinder is forced to the control valve and back to the pump reservoir.
The steering unit includes a right turn pressure line and a left turn pressure line both leading from the pinion housing and extending to opposite sides of the piston in the power cylinder. During a right turn, for example, power steering fluid is forced under pressure through the right turn pressure line and returned to the control valve through the left turn pressure line.
There are a number of oil seals in the pinion portion of the housing which can be subject to leakage--the lower pinion seal in the lower portion of the pinion housing, and four upper pinion seals on the valve spool. In addition, there are three oil seals in the rack portion of the steering unit--the bulkhead seal, the rack seal, and the high pressure inner rack seal.
There is a need for a technique to test all of these oil seals for leakage while the power steering unit is removed from the vehicle. In this way, it would be possible to accurately isolate the areas where oil seal leakage is present, not to mention the advantages of reduced labor and time and lower expense in conducting tests with the steering unit removed from the vehicle.